When performing digital transmission at very high data rates, such as that mentioned above, it is desirable for each train of binary 0 or 1 pulses constituting a signal should have no DC component and should have very low energy density in the low frequency portion of the spectrum. One of the problems lies in the optical transmitter and receiver heads which are used having a low frequency cutoff. Another lies in the need to use low frequency transmission both for service information and for remote surveillance information, and the need to avoid them interfering.
One known method of satisfying the above requirements is described in U.S. Pat. No. 3,405,235 in which a digital train is transmitted in parallel over a plurality of paths as a series of parallel-transmitted words subjected to a block code designed so that the accumulated disparity: ##EQU2## remains low, where n is the number of the most recently transmitted word, and Sk is the "sum" of the k-th word such that Sk equals the number of 1-bits minus the number of 0-bits in the k-th word.
The low accumulated disparity coding method as described in said U.S. patent is as follows:
if .SIGMA..sub.n-1 and Sn have the same sign, then the word of sum Sn is inverted, i.e. its 0-bits are transformed into 1-bits and vice-versa; and
if .SIGMA..sub.n-1 and Sn are of opposite sign, the word of sum Sn is transmitted unchanged.
Prior circuits for implementing such low accumulated disparity codes use either programmable memories which cannot be used at the required high data rate because of their excessive access times, or else they use one or more adders which cannot be used either since their maximum operating frequency is too low and their propagation time is too high to be used at such high data rates, e.g. data rates of about 140 megabits/second.